/*
 * Copyright 2012 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */
package io.netty.handler.codec;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandler.Sharable;
import io.netty.channel.ChannelHandlerContext;
import io.netty.util.internal.ObjectUtil;

import java.nio.ByteOrder;
import java.util.List;

/**
 * An encoder that prepends the length of the message. The length value is
 * prepended as a binary form.
 * <p>
 * For example, <tt>{@link LengthFieldPrepender}(2)</tt> will encode the
 * following 12-bytes string:
 * 
 * <pre>
 * +----------------+
 * | "HELLO, WORLD" |
 * +----------------+
 * </pre>
 * 
 * into the following:
 * 
 * <pre>
 * +--------+----------------+
 * + 0x000C | "HELLO, WORLD" |
 * +--------+----------------+
 * </pre>
 * 
 * If you turned on the {@code lengthIncludesLengthFieldLength} flag in the
 * constructor, the encoded data would look like the following (12 (original
 * data) + 2 (prepended data) = 14 (0xE)):
 * 
 * <pre>
 * +--------+----------------+
 * + 0x000E | "HELLO, WORLD" |
 * +--------+----------------+
 * </pre>
 */
@Sharable
public class LengthFieldPrepender extends MessageToMessageEncoder<ByteBuf>
{

    private final ByteOrder byteOrder;

    private final int lengthFieldLength;

    private final boolean lengthIncludesLengthFieldLength;

    private final int lengthAdjustment;

    /**
     * Creates a new instance.
     *
     * @param lengthFieldLength the length of the prepended length field. Only
     *        1, 2, 3, 4, and 8 are allowed.
     *
     * @throws IllegalArgumentException if {@code lengthFieldLength} is not 1,
     *         2, 3, 4, or 8
     */
    public LengthFieldPrepender(int lengthFieldLength)
    {
        this(lengthFieldLength, false);
    }

    /**
     * Creates a new instance.
     *
     * @param lengthFieldLength the length of the prepended length field. Only
     *        1, 2, 3, 4, and 8 are allowed.
     * @param lengthIncludesLengthFieldLength if {@code true}, the length of the
     *        prepended length field is added to the value of the prepended
     *        length field.
     *
     * @throws IllegalArgumentException if {@code lengthFieldLength} is not 1,
     *         2, 3, 4, or 8
     */
    public LengthFieldPrepender(int lengthFieldLength,
            boolean lengthIncludesLengthFieldLength)
    {
        this(lengthFieldLength, 0, lengthIncludesLengthFieldLength);
    }

    /**
     * Creates a new instance.
     *
     * @param lengthFieldLength the length of the prepended length field. Only
     *        1, 2, 3, 4, and 8 are allowed.
     * @param lengthAdjustment the compensation value to add to the value of the
     *        length field
     *
     * @throws IllegalArgumentException if {@code lengthFieldLength} is not 1,
     *         2, 3, 4, or 8
     */
    public LengthFieldPrepender(int lengthFieldLength, int lengthAdjustment)
    {
        this(lengthFieldLength, lengthAdjustment, false);
    }

    /**
     * Creates a new instance.
     *
     * @param lengthFieldLength the length of the prepended length field. Only
     *        1, 2, 3, 4, and 8 are allowed.
     * @param lengthAdjustment the compensation value to add to the value of the
     *        length field
     * @param lengthIncludesLengthFieldLength if {@code true}, the length of the
     *        prepended length field is added to the value of the prepended
     *        length field.
     *
     * @throws IllegalArgumentException if {@code lengthFieldLength} is not 1,
     *         2, 3, 4, or 8
     */
    public LengthFieldPrepender(int lengthFieldLength, int lengthAdjustment,
            boolean lengthIncludesLengthFieldLength)
    {
        this(ByteOrder.BIG_ENDIAN, lengthFieldLength, lengthAdjustment,
                lengthIncludesLengthFieldLength);
    }

    /**
     * Creates a new instance.
     *
     * @param byteOrder the {@link ByteOrder} of the length field
     * @param lengthFieldLength the length of the prepended length field. Only
     *        1, 2, 3, 4, and 8 are allowed.
     * @param lengthAdjustment the compensation value to add to the value of the
     *        length field
     * @param lengthIncludesLengthFieldLength if {@code true}, the length of the
     *        prepended length field is added to the value of the prepended
     *        length field.
     *
     * @throws IllegalArgumentException if {@code lengthFieldLength} is not 1,
     *         2, 3, 4, or 8
     */
    public LengthFieldPrepender(ByteOrder byteOrder, int lengthFieldLength,
            int lengthAdjustment, boolean lengthIncludesLengthFieldLength)
    {
        if (lengthFieldLength != 1 && lengthFieldLength != 2
                && lengthFieldLength != 3 && lengthFieldLength != 4
                && lengthFieldLength != 8)
        {
            throw new IllegalArgumentException(
                    "lengthFieldLength must be either 1, 2, 3, 4, or 8: "
                            + lengthFieldLength);
        }
        ObjectUtil.checkNotNull(byteOrder, "byteOrder");

        this.byteOrder = byteOrder;
        this.lengthFieldLength = lengthFieldLength;
        this.lengthIncludesLengthFieldLength = lengthIncludesLengthFieldLength;
        this.lengthAdjustment = lengthAdjustment;
    }

    @Override
    protected void encode(ChannelHandlerContext ctx, ByteBuf msg,
            List<Object> out) throws Exception
    {
        int length = msg.readableBytes() + lengthAdjustment;
        if (lengthIncludesLengthFieldLength)
        {
            length += lengthFieldLength;
        }

        if (length < 0)
        {
            throw new IllegalArgumentException(
                    "Adjusted frame length (" + length + ") is less than zero");
        }

        switch (lengthFieldLength)
        {
            case 1:
                if (length >= 256)
                {
                    throw new IllegalArgumentException(
                            "length does not fit into a byte: " + length);
                }
                out.add(ctx.alloc().buffer(1).order(byteOrder)
                        .writeByte((byte) length));
                break;
            case 2:
                if (length >= 65536)
                {
                    throw new IllegalArgumentException(
                            "length does not fit into a short integer: "
                                    + length);
                }
                out.add(ctx.alloc().buffer(2).order(byteOrder)
                        .writeShort((short) length));
                break;
            case 3:
                if (length >= 16777216)
                {
                    throw new IllegalArgumentException(
                            "length does not fit into a medium integer: "
                                    + length);
                }
                out.add(ctx.alloc().buffer(3).order(byteOrder)
                        .writeMedium(length));
                break;
            case 4:
                out.add(ctx.alloc().buffer(4).order(byteOrder)
                        .writeInt(length));
                break;
            case 8:
                out.add(ctx.alloc().buffer(8).order(byteOrder)
                        .writeLong(length));
                break;
            default:
                throw new Error("should not reach here");
        }
        out.add(msg.retain());
    }
}
